1. Field of the Invention
The present invention relates to a GaN group compound semiconductor device, e.g., a semiconductor laser diode, and a method for producing the same.
2. Description of the Related Art
GaN group compound semiconductors, generally defined by the formula In.sub.x Ga.sub.y Al.sub.z N (where x+y+z=1; and 0.ltoreq.x, y, z.ltoreq.1), have a large energy band gap and high thermal stability, and therefore are regarded as a class of promising material systems adaptable to various applications, including light emitting devices and high-temperature devices. Among others, LEDs (light emitting diodes) composed essentially of such materials, which provide luminous intensity on the order of several candelas (cd) in wavelengths ranging from blue to green, have already been developed and applied to practical use. Future research and development activities will be directed to the achievement of a full color system (i.e., by providing illumination at still longer wavelengths) or laser diodes (LD) composed essentially of such materials.
FIG. 1 is a schematic cross-sectional view showing a conventional structure employed as a p-side electrode for the aforementioned devices. As shown in FIG. 1, the p-side electrode includes a metal layer 602 (composed essentially of Ni) formed on a p-GaN layer 601 functioning as a p-contact layer, as well as an intermediate layer 604 (composed essentially of GaN and Ni). The intermediate layer 604 is formed by performing annealing at 500.degree. C. for 10 minutes in a nitrogen atmosphere while the Ni layer 602 is in contact with the p-GaN layer 601. Furthermore, a surface electrode layer 603 is layered on the Ni layer 602 for facilitating wire bonding and/or device mounting. A typical material employed for the surface electrode layer 603 is Au.
If the Ni layer 602 is in direct contact with the p-GaN layer 601 without the intermediate layer 604 interposed therebetween, a schottky barrier (energy barrier) Es formed on the valence band side of an interface S between the Ni layer 602 and the p-GaN layer 601 has a height as shown in the graph of FIG. 2. The schottky barrier (energy barrier) Es is reduced by the incorporation of the intermediate layer 604 as shown in the graph of FIG. 3.
However, a GaN group p-side electrode structure according to the aforementioned conventional technique has a problem in that the electrode has unstable ohmic properties as well as a relatively high contact resistivity of about 10.sup.-2 .OMEGA. cm.sup.2 to about 10.sup.-3 .OMEGA. cm.sup.2. The aforementioned conventional technique cannot attain a contact resistivity typically required for a p-side electrode of a semiconductor laser, i.e., about 5.times.10.sup.-4 .OMEGA. cm.sup.2 or less.